1. Field of the Invention
This invention relates to heat dissipation of a semiconductor device or electronic module, and in particular to a method and means for accomplishing such dissipation and reducing stresses caused by thermal gradients by employing an electronic module cap and heat exchanger as a single structure, and a semi-molten material between the IC chip and its associated heat exchanger to melt and solidify in the IC chip operational temperature range in order to release and absorb heat energy and create a low thermal resistance path to the heat exchanger.
2. Description of Related Art
Technological advancements in semiconductor device design and fabrication have significantly increased integrated circuit chip power requirements, and these requirements continue to climb. In order to accommodate these enhanced power requirements, it has become necessary to find more efficient ways to dissipate heat through the electronic packages and their associated heat exchangers.
Current electronic module designs do not efficiently dissipate heat energy at rates beyond power levels of 25 to 50 Watts. Beyond these levels expensive methods of water cooling are necessary to minimize the external thermal resistance and maximize the thermal gradient, thus enhancing heat flow. The power dissipation is inherently limited by both internal and external thermal resistance to heat flow. Thermal pastes currently in use as low thermal resistant conductors between an IC chip and its heat exchanger are adequate to satisfy the thermal dissipation requirements at the present state of the art, however, they are inadequate for the much higher power allotments being contemplated for future designs. Although there are many materials, especially metals, that have excellent thermal conductivity satisfying the thermal dissipation requirements, most of the metals in the solid state induce intolerable stresses in the multi-chip modules during thermal cycling. The conductive material used for this heat transfer is typically placed between the IC chip and the heat exchanger, and consequently may be exposed to considerably different thermal expansion coefficients (TECs) and/or different temperatures due to thermal gradients. The result is IC chip cracking, unacceptable C-4 solder deformation, and excessive fatigue damage leading to early failure.
There is also the associated problem of operational thermal cycling or mini-cycles that are common among devices employing power saving schemes, such as CMOS technology parts. The alternating power demands by the IC chip exacerbate the power dissipation requirements and enhance device fatigue. Thus, in order to accommodate a new generation of devices and establish a more efficient thermal transfer from the IC chip to its heat exchanger, the effects of mini-cycles must also be attenuated.
To prevent an IC chip from exceeding its prescribed operating temperature, it is necessary to maximize the heat flow across the interface to the chip""s heat exchanger such that heat may be effectively conducted away from the chip. A key concern with high heat generating chips is the achievement of very low and stable chip thermal contact resistance. This requires the use of a thermal dissipation technique that embodies thermal and mechanical properties such that uniform contact loading and low interface thermal resistance are continuously maintained. The prior art has considered a number of techniques towards this objective. Most can be classified as one of either a solid metallurgical bond, a microstructural or structural contact, a demountable liquid or grease interface, or a combination of these types.
In U.S. Pat. No. 5,170,930 issued to Dolbear, et al. on Dec. 15, 1992, entitled xe2x80x9cLIQUID METAL PASTE FOR THERMAL AND ELECTRICAL CONNECTIONSxe2x80x9d, a thermally and electrically conductive paste is used for cooling electronics. The conductive and compliant paste are connected between the IC chip surface and the heat exchanger surface. It consists of an equilibrium mixture of liquid metal with particulate solid constituents, such as particles or fibers. The materials added to the liquid metal as well as their compositions are selected such that a paste is formed instead of a permanent solid. Various types of liquid metals are suggested, including gallium, indium, mercury, cadmium and bismuth. Also, numerous powder additives are employed such as alumina, aluminum, aluminum nitride, chromium, gold, lead, and silicon, to name a few. The application of the paste is for thermal conduction purposes. As such, the paste does not undergo a change of state, i.e., melt and re-solidify under the IC chip operational conditions. The mixture does not provide the melting and solidifying properties in the IC chip operational temperature range for which the material must have the properties of releasing or absorbing sufficient heat to counteract the effects of mini-cycles.
In U.S. Pat. No. 5,325,265 issued to Turlik, et al. on Jun. 28, 1994, entitled xe2x80x9cHIGH PERFORMANCE INTEGRATED CIRCUIT CHIP PACKAGExe2x80x9d, cushions formed of thermally conductive low melting point material are placed between the heat sink and the IC chip for the transfer of heat from the chip to the heat exchanger. The cushion is sufficiently thick to be able to absorb movement (dimensional variations) between the IC chip and the heat exchanger during thermal cycling, yet sufficiently thin to act as a thermal conductor between the chip and the heat exchanger. In forming this package, the stress absorbing cushions are designed to have the lowest melting point or deformation temperature of all components used. Once brought together, the entire assembly is heated above the melting temperature of the cushion. The cushions melt and reflow to form conformal cushions between the heat exchanger and the IC chip accommodating the separation irregularities. The preferred material for the cushion is indium or an alloy of indium. Once again, this application is for thermal conduction purposes. The solder is not designed to melt and re-solidify under the IC chip operational temperature range which would absorb and reject heat. Thus, this art does not have the capacity for heat generation and subtraction during the rise and fall of temperature due to mini-cycles. As such, the mini-cycle thermal excursions are neither suppressed or attenuated.
Lastly, in U.S. Pat. No. 4,607,277 issued to Hassan et al. on Aug. 19, 1986, entitled xe2x80x9cSEMICONDUCTOR ASSEMBLY EMPLOYING NONEUTECTIC ALLOY FOR HEAT DISSIPATIONxe2x80x9d, an alloy was employed to form a low thermal resistance bridging interface between the surface of an IC chip and the surface of a heat exchanger. The alloy has a solidus-liquidus temperature range such that the solidus is slightly below the maximum operating temperature of the chip, and thus has the capability to reestablish and maintain the interface at a low thermal resistance if stressed during circuit operation. The alloy, however, which is preferably a composition of bismuth, lead, tin and indium, initially only makes contact with the surface of the chip at spaced points between voids. During IC package assembly, the temperature of the device is raised to the melting range of the alloy. The bismuth alloy then melts and conforms under viscous flow conditions to fill the void spaces between the chip and the heat exchanger. Disruptions due to dimensional variations, differential thermal expansions, and impact load effects will increase the thermal resistance due to the reoccurrence of voids. Thus, the temperature will rise into the melting range of the alloy, thereby recovering the low thermal resistance. In effect, a self-healing process of the alloy melting to conform to the chip surface occurs so that the contact thermal resistance is maintained at a low level. Similar to the above referenced inventions, this prior art also has the objective of dissipating heat more efficiently by minimizing interfacial thermal resistance. Here, it is done with the use of molten solder. The solder, however, remains a distance away from the chip and would not be able to significantly attenuate the temperature cycling due to mini-cycles even if the solder were suitable and had the requisite thermodynamic properties to meet the requirements of power dissipation in the proper temperature range. Thus, the patent is strictly one for improving heat conduction through the improvement of the thermal resistance path.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide a method and apparatus for the removal of heat from an integrated circuit chip to ambient air that attenuates the effects of thermal mini-cycles associated with IC chip operation.
It is another object of the present invention to provide a method and means for removing heat from an integrated circuit chip without incurring thermally induced shear stress on the chip or the accompanying substrate.
A further object of the invention is to provide a low thermal resistance path between the IC chip and its associated heat exchanger.
It is yet another object of the present invention to provide an IC chip package in which thermal mismatches between the chip, its associated heat exchanger, and an interfacing thermal conducting material are minimized.
Yet another object of the present invention is to provide a more efficient way to dissipate heat from an integrated circuit chip through its associated heat exchanger and the joint bond there between.
Still other advantages of the invention will in part be obvious and will in part be apparent from the specification.
The above and other advantages, which will be apparent to one of skill in the art, are achieved in the present invention which is directed to, in a first aspect, a method of making a thermally conductive interface for an electronic module comprising: providing an integrated circuit chip with a top and bottom surface, and an operational temperature range between a highest temperature and a lowest temperature, wherein the integrated circuit chip generates thermal cycles within the operational temperature range; and applying a bonding composition to the integrated circuit chip such that the bonding composition has a solidus-liquidus temperature range that encompasses the integrated circuit chip operational temperature range, wherein the solidus temperature is less than or equal to the integrated circuit chip operational temperature range lowest temperature and the liquidus temperature is greater than or equal to the integrated circuit chip operational temperature highest temperature, and heat generated by each of the thermal cycles is partially or totally absorbed by the melting of a portion of the bonding composition that is in a solid phase, and heat lost by each of the thermal cycles is compensated by heat released from the solidifying of a portion of the bonding composition that is in a liquid phase. Additionally, the method further comprises providing a cap-heat exchanger cover with a bottom surface, including a cap cover portion and a heat exchanger cover portion formed in a single cover piece, wherein the bottom surface of the heat exchanger cover portion is attached to the integrated circuit chip top surface. However, the method may also comprise a cap-heat exchanger cover with a bottom surface, where the cap cover portion is attached to the heat exchanger cover portion.
The method further comprises providing a substrate with a top surface such that the bottom surface of the integrated circuit chip is connected to the substrate top surface, and the bottom surface of the cap cover portion is attached to the substrate top surface. The substrate has seal bands on the top surface to contain the bonding composition and provide an hermetic seal for the electronic module.
The bonding composition comprises: a preform Bixe2x80x94In composition of 25% Bi by weight, with a liquidus temperature of about 100xc2x0 C.; a preform Bixe2x80x94In eutectic of 34% Bi by weight, with a liquidus temperature of about 72xc2x0 C.; or, preform solder materials that melt at or below room temperature including:
a) Gaxe2x80x94In composition of 70% In by weight, with a eutectic temperature of about 16.5xc2x0 C. and a liquidus of about 90xc2x0 C.;
b) Gaxe2x80x94Zn composition of 85% Ga by weight, with a eutectic temperature of about 29.8xc2x0 C. and a liquidus of about 110xc2x0 C.; or,
c) Gaxe2x80x94Sn composition of 40% Sn by weight, with a eutectic temperature of about 20xc2x0 C. and a liquidus of about 110xc2x0 C.
The method further includes a thin solder wetable film structure over the top surface of the integrated circuit chip, the bottom surface of the heat exchanger cover portion, the bottom surface of the cap cover portion, and the top surface of the substrate, to achieve bonding during the bonding composition reflow. The thin film structure is preferably comprised of Crxe2x80x94Nixe2x80x94Au films deposited or evaporated on each of the surfaces.
The bonding composition can be affixed within a cavity within the heat exchanger bottom surface. The cap-heat exchanger cover is comprised of a ceramic material 1 mm to 2.5 mm thick, such that the cover is compliant, conforming to vertical displacements caused by different thermal expansions of materials. The cap cover portion may also be connected to the substrate top surface, and to the seal bands on the substrate top surface, through a Pbxe2x80x94Sn alloy preform compatible with the thermal hierarchy associated with the method.
The present invention is directed to, in a second aspect, a method of making a thermally conductive interface for an electronic module comprising:
providing a substrate with a top surface, having seal bands on the top surface;
providing an integrated circuit chip with a top and bottom surface, and operational within a temperature range between a highest temperature and a lowest temperature, the bottom surface connected to the substrate top surface, wherein the integrated circuit chip generates thermal cycles within the operational temperature range;
providing a cap-heat exchanger cover with a bottom surface, comprising a cap cover portion and a heat exchanger cover portion formed in a single cover piece, wherein the bottom surface of the cap cover portion is attached to the substrate top surface, and the bottom surface of the heat exchanger cover portion is attached to the integrated circuit chip top surface;
connecting the top surface of the integrated circuit chip to the bottom surface of the heat exchanger cover with a solder composition, wherein the solder composition has a solidus-liquidus temperature range that encompasses the integrated circuit chip operational temperature range, wherein the solidus temperature is less than or equal to the integrated circuit chip operational temperature range lowest temperature and the liquidus temperature is greater than or equal to the integrated circuit chip operational temperature highest temperature, and heat generated by each of the thermal cycles is partially or totally absorbed by the melting of a portion of the solder composition that is in a solid phase, and heat lost by each of the thermal cycles is compensated by heat released from the solidifying of a portion of the solder composition that is in a liquid phase; and
connecting the bottom surface of the cap cover to the top surface of the substrate.
The method further comprises a diffusion barrier, lined on the cover bottom surface, to limit ingress of oxygen, solder oxidation, and solder outflow in non-hermetically sealed and semi-hermetically sealed electronic modules. A thin film structure comprised of Crxe2x80x94Nixe2x80x94Au films is deposited or evaporated on each of the solder contacting surfaces. Also, the connections are performed by applying heat to the electronic module above solder melting temperatures.
In a third aspect, the present invention is directed to a method for connecting two surfaces of an electronic module comprising:
providing a substrate with a top surface, having seal bands on the top surface;
providing an integrated circuit chip with a top and bottom surface, and an operational temperature range between a highest temperature and a lowest temperature, the integrated circuit chip bottom surface attached to the substrate top surface;
providing a cover over the integrated circuit chip and the substrate, comprising a cap portion with a bottom surface and a heat exchanger portion with a bottom surface; and
attaching the cover to the substrate and the integrated circuit chip, the attachment comprising:
i) depositing or evaporating an alloy with a defined melting point on the substrate top surface, the cap cover portion bottom surface, the heat exchanger portion bottom surface, and the substrate seal bands;
ii) heating the alloy above the melting point;
iii) applying a solder composition preform between the integrated circuit chip top surface and the heat exchanger cover portion bottom surface, wherein the solder composition has a solidus-liquidus temperature range encompassing the integrated circuit chip operational temperature range;
iv) applying a Pbxe2x80x94Sn solder preform between the cap cover portion bottom surface and the substrate top surface; and
iv) heating the electronic module above the melting point of the solder preforms.
This method further comprises providing an elastomeric ring between the cover and the substrate top surface to non-hermetically sealed and semi-hermetically sealed electronic modules, such that the ring eliminates solder outflow and prohibits environmental degradation to soldered surfaces.
In a fourth aspect, the present invention is directed to an apparatus for maximizing thermal conduction and eliminating the adverse effects of operational thermal cycling comprising:
an integrated circuit chip having a top surface and a bottom surface, and operational within a temperature range between a highest temperature and a lowest temperature; and
a bonding composition on a surface of the integrated circuit chip, the bonding composition having a solidus-liquidus temperature range encompassing the integrated circuit chip operational temperature range, such that the bonding composition is in a semi-molten state when power is applied to the integrated circuit chip and the chip is operating within the chip operational temperature range.
The apparatus further comprises a substrate with a top surface, having seal bands on the top surface, wherein the integrated circuit chip bottom surface is attached to the substrate top surface.
Additionally, the apparatus comprises, a cover attached to the substrate top surface and the integrated circuit chip top surface, comprised of a ceramic cap cover portion and a heat exchanger cover portion, wherein the cover is mounted as a single piece over the integrated circuit chip and the substrate, the heat exchanger cover portion having a bottom surface over the integrated circuit chip top surface.
The apparatus further comprises a solder wetable thin film structure over the top surface of the integrated circuit chip, the bottom surface of the heat exchanger cover portion, the bottom surface of the cap cover portion, and over the top surface of the substrate.
The apparatus may also be comprised of a cap cover that is ceramic. The bonding composition is preferably on the order of 13 to 17 mils in thickness.
Additionally, the apparatus further comprises a diffusion barrier, for use with non-hermetically sealed or semi-hermetically sealed electronic modules, of a polymeric ring fitted within a groove in the cover.
The apparatus includes a thin film structure that is comprised of a deposition or evaporation of Crxe2x80x94Nixe2x80x94Au layers. This thin film structure includes a Cr layer 100 nm thick, a Ni layer 1000 nm thick, and a Au layer 50 nm thick.
The apparatus may also include a bonding composition used at the eutectic composition wherein the composition will melt completely at the eutectic temperature of the alloy.
The present invention, in a fifth aspect, is directed to a method of using an electronic module for maximizing thermal conduction and eliminating the adverse effects of operational thermal cycling comprising:
providing an integrated circuit chip having a top surface and a bottom surface, and operational within a temperature range, and a bonding composition on of the surfaces, the bonding composition having a solidus-liquidus temperature range encompassing the integrated circuit chip operational temperature range;
applying power to the integrated circuit chip; and,
cycling temperature within the integrated circuit chip a maximum and minimum temperature within the operational temperature range such that the bonding composition remains in a semi-molten state when power is applied to the integrated circuit chip, such that heat energy is absorbed by a change of state of the bonding composition from a solid state to a liquid state, and heat energy is delivered by a change of state of the bonding composition from a liquid state to a solid state.
The method further comprises removing heat energy from the integrated circuit chip through a heat exchanger attached to the chip by the bonding composition.